As part of scientific activities of the Middle Atmosphere Program in Japan, the direct sampling of trace gases and aerosols in the lower stratosphere has been planned by using Merlin-IV aircraft. Most of the flights have been performed during winter, since Merlin-IV can reach the lower stratosphere only during cold seasons at latitudes of 36°–40°N. During the sampling flights, concurrent with the aerosol sampling, O3 mixing ratios were measured simultaneously from aircraft by using a modified airborne ozonesonde. The O3 mixing ratio is used as tracer of stratospheric air and, together with radiosonde records of the Japan Meteorological Agency's local weather stations is a good indicator of the height of the tropopause along the flight track.